Electrographic devices in which coordinate pair signals are generated by manual positioning of a locator or the like upon an operative surface have become subjects of increasing popularity both to industry and within the consumer market. Industrial demands for the devices have occurred in conjunction with the evolution of computer graphics, computer aided design, and computer aided manufacturing systems. As these systems have been improved, a need has been observed for combining hte digitizer funcction with a visual readout such that the operational aspects of vision and information input may be combined to perform in concert before the operator. In the latter regard, an outgrowth of this combination of htese function sresides in the electronic notepad or hte like wherein business forms and the like established at a display output may be "filled in" employing a stylus or cursor and in conjunction with character recognition software. For these advanced aspects of computerized graphics information systems to achieve practicality, the digitizer components of the system must be fabricable on a practical basis with minimal electronic supporting bulk, high noise immunity, high resolution and accuracy and at cost levels commensurate with the relatively higher volume applications contemplated.
The operation of a classic digitizer or graphics tablet has generally involved the utilization by an operator of a stylus or tracer locating device representing a writing instrument which is positioned upon the operational surface and moved across it in some electrical association. The electrographic device responds to the position of this locating stylus or tracer to generate paired analog coordinate signals which are digitized and conveyed to a computer facility. For electronic notepad applications, the computer respojndst othe paired coordinate signals to generate a pixel at a display positioned immediately adjacent the digitizer surface and at the location of the stylus or tracer. As is apparent, high resolution capabilities are required for such applications.
Early approaches to digitizer structures looked to arrangements wherein a grid formed of two spaced arrays of mutually, orthogonally disposed fine wires are embedded in an insulative carrier. One surface of this structure serves to yieldably receiuve a stylus input which yielding causes the grid components to intersect and read out coordinate signals. More recent and improved approaches to achieving readouts have been accomplished through resort to a capacitive coupling of the stylus or locating instrument with the position responsive surface to generate paired analog coordinate signals. Such capacitive coupling can be carried out either with a grid layer which is formed of spaced linear arrays of conductors or through resort to the use of an electrically resistive material layer or coating.
Particularly where applications of combining the digitizer surface with a visual readout are contemplated, the provision of the digitizer surface as a somewhat continuous resistive material shows immediate apparent advantage. Such transparent coatings additionally may be employed with digitizer tablets which are placed over drawings, photographic material, or the like for tracing profiles and generating computer data corresponding therewith.
A variety of technical problems have been encountered in the development of an effective resistive coating type digitizer technology, one of which concerns the non-uniform nature of the coordinate readouts received from the surfaces. Generally, precise one-to-one correspondence or linearity is required between the actual stylus or tracer position and the resultant coordinate signals. Because the resistive coatings cannot be practically developed without local resistance (thickness) variations the nonlinear aspects of the otherwise promising approach have required a considerable amount of inventigation and development. An earlier development in this regard is described by Turner in U.S. Pat. No. 3,699,439 entitled "Electrical Probe-Position Responsive Apparatus and Method" issued Oct. 17, 1972, and assigned in common herewith. This approach uses a direct current form of input to the resistor surface from a hand-held stylus, the tip of which is physically applied to the resistive surface. Schlosser, et al. in U.S. Pat. No. 4,456,787, entitled "Electrographic System and Method", issued June 26, 1984, also assigned in common herewith, describes the employment of an a.c. input signal in conjunction with such devices as well as the signal treatment of the resulting coordinate pair output signal. A voltage waveform zero crossing approach has been suggested by Turner to improve resolution in U.S. Pat. No. 4,055,726 entitled "Electrical Position Resulting by Zero-Crossing Delay" issued Oct. 25, 1977, and assigned in common herewith. Kable, in U.S. Pat. No. 4,600,807, issued July 15, 1986, and assigned in common herewith, describes a successful signal treatment technique for transparent digitizer systems. In general, this approach utilizes a plurality of switches along the four coordinate borders of the tablet structure. An a.c. drive signal is applied from one border, while the opposite border is retained at ground for a given coordinate readout, for example in the x-axis direction. Plus and minus values are developed for generating x-coordinate pairs as well as y-coordinate pais and during the evaluation process, those switches aligned along the borders not being used at ground or as drivers are retained in a "floating" condition. Thus the switching employed with such digitizing approaches exhibit three states for a given coordinate generating operation. In general, the utilization of a third or floating state with these switches has been the subject of some noise generation. Inasmuch as the switches are receptive to flowing currents within the digitizer surfaces during such time as they are in a floating state, any thermal responses ofthe switching components themselves will be reflected in teh overall signal process. Avoidance of this state for such switches as well as the relatively large requisite number of switches now employed in digitizer assemblages will be of considerable advantage in achieving improved operation and desired simplicity.
Substantially improved accuracies for the resistive surface type digitizing devices have been achieved through a correction procedure wherein memory retained correction data are employed with the digitizer such that any given pair of coordinate signals are corrected in accordance with data collected with respect to each digitizer resitive surface unit during its manufacture. With such an arrangement, the speed of correction is made practical and the accuracy of the devices is significantly improved. Such correction process is described by Nakamura, et al., in U.S. Pat. No. 4,650,926, issued Mar. 17, 1987, and assigned in common herewith.
In order to avoid interference from externally generated noise, hand effects, and the like, the resistivity for transparent digitizers preferably falls within predetermined acceptable ranges, for example, between 400 and 3,000 ohms per square. To achieve the higher levels of resistivity thus desired, very thin resistive coatings, for example of indium tin oxide (ITO) have been employed. However, it has been observed that, over a period of time, surface effects and the like will affect the resistivity values of a given tablet occasioning an unwanted "drift" in such value to affect long term accuracy. To improve the long term stability of the coatings, thicker coatings have been employed in combination with discontinuities in the layer itself as described by Kable, et al., in U.S. Pat. No. 4,665,283, issued May 12, 1987, and assigned in common herewith. Improvements in performance also have been achieved through the utilization of angular shaped electrodes at corner positions as well as a conductive band or band of enhanced conductivity which is positioned intermediate the outer periphery of the digitizer device and the active area thereof as described by Nakamura, et al., in U.S. Pat. No. 4,649,232 entitled "Electrographic Apparatus" and assigned in common herewith.
Improvements in the locators or pick-up devices themselves utilized with digitizers have been evolved to enhance overall performance of the systems. For example, an improved tracer or cursor is described by Kable et al. in U.S. Pat. No. 4,707,572, entitled "Tracer for Electrographic Surfaces" issued Nov. 17, 1987, assigned in common herewith. Similarly, Kable describes an improved stylus structure in U.S. Pat. No. 4,695,680, entitled "Stylus for Position Responsive Apparatus Having Electrographic Application" issued Sept. 22, 1987, and assigned in common herewith.